Red-emitting material for cathodoluminescent screens

ABSTRACT

A phosphor of europium activated lithium indate provides a redemitting luminescent material of high brightness and good chromaticity and stability under electron beam excitation. The concentration of europium is relatively low in comparison with other similarly activated phosphors.

United States Patent RED-EMITTING MATERIAL FOR CATHODOLUMINESCENT SCREENS 2 Claims, 2 Drawing Figs.

Int. Cl

Field of Search [56] References Cited UNITED STATES PATENTS 3,497,458 2/1970 Schuil 252/301.4 R 3,301,791 1/1967 Brixner 252/3014 OTHER REFERENCES Blasse et al., On the Eu Fluorescence in Mixed Metal Oxides- Journal of Chemical Physics, Vol. 45, No. 9, Nov. 1, 1966, pages 3,327- 3,332

Primary ExaminerRobert D. Edmonds Attorneys-C. Cornell Remsen, Jr., Rayson P. Morris, Percy P. Lantzy, Philip M. Bolton and Isidore Togut ABSTRACT: A phosphor of europium activated lithium indate provides a red-emitting luminescent material of high brightness and good chromaticity and stability under electron beam excitation. The concentration of europium is relatively low in comparison with other similarly activated phosphors.

RED-EMITTING MATERIAL FOR CATI-IODOLUMINESCENT SCREENS BACKGROUND OF THE INVENTION describes the various advantages and shortcomings of some of these materials employed in color television tubes and particularly the improvement in the red color emission and brightness provided by YVO,:Eu. While Y,O :Eu has a higher luminous efficiency, it has an emission color on the orange side of the red portion of the frequency spectrum. YVO,:Eu, however, has a sufficiently high efficiency with improved red characteristics. In general, europium-activated materials provide a greater color stability and linearity of response with changes of electron beam current or brightness, in comparison with known sulfide materials which change colors somewhat under varying conditions.

The presently known europium phosphor compounds however require at least 4-5 percent europium for best color results and tend to change toward orange at lower concentrations. The yttrium oxysulfide compound, in addition, requires a costly and difficult preparation process. Another phosphor composed of europium activated lithium indate, Lilno zEu, was previously prepared and examined under ultraviolet excitation, but was considered as having too low an efficiency for satisfactory performance. This material was one of many europium activated phosphors reported in an article entitled On the Eu Fluorescence in Mixed Metal Oxides. V. The Eu-Fluorexence in the Rocksalt Lattice," by G. Blasse and A. Bril, published in the Journal of Chemical Physics, Nov. l, 1966. The present inventor however has discovered that this composition, made of particular mixtures of materials in specified proportions, provides a highly efficient red-emitting cathodoluminescent phosphor.

SUMMARY OF THE INVENTION It is therefore the primary object of the present invention to provide a novel red-emitting luminescent material of high brightness and good color response and stability for use in color cathode-ray tubes. This is achieved with europium-activated lithium indate, LilnO zEu, as the red-emitting phosphor forming portions of the luminescent screen coating which provides color displays under electron beam excitation. The material is prepared by firing a particular mixture of lithium, indium, and europium compounds which provide highly efficient stable emission in the red region of the visible spectrum with relatively small proportions of europium. The details of the invention will be more fully understood and other objects and advantages will become apparent by reference to the following description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic representation of a cathode-ray tube having a luminescent screen including the novel phosphor of thc invention, and

FIG. 2 shows a response curve of relative emission intensity in a hand of wavelengths in the red region of the visible spec- Hunt.

DESCRIPTION OF THE PREFERRED EMBODIMENT The europium-activated lithium indate phosphor is prepared by a solid-state reaction between a lithium compound that readily decomposes at specified firing temperatures in the presence of an indium compound, and includes such materials as lithium nitrate, LiNO,, lithium carbonate, LiCO;, lithium acetate, LiC,H,O,, or lithium oxalate, Li C,O,. The indium compound may include indium oxide, In O,, indium nitrate, In(NO,),, indium acetate, ln(C,H 0,) indium oxalate, In,(C,O or other indium compounds that decompose into the oxide in the presence of a lithium compound. The mixture includes a europium compound such as europium oxide, Eu,O europium nitrate, Eu(NO,);,, europium acetate, Eu(C H Oz)3, or europium oxalate, Eu2(C2O4).-t that will combine with lithium indate. LiNO and other lithium compounds have been found to give better results than Li CO probably due to the fact that they decompose to the oxide more readily than Li,CO The latter can also provide good results by using longer firing times and/or higher firing temperatures.

The starting materials in the form of powders are mixed together and fired at from 900-l ,300" C. in a nonreducing atmosphere such as air, oxygen, or oxygen with an inert gas. Higher temperatures may also be used, although some of the U 0 and in,0, may be lost due to volatilization. The firing time can vary from 1 hour or less to many hours and multiple firings with interposed grinding steps may be used to provide a product having a more complete reaction. The final compounds are in the form of powders and crystals. The technique is similar to that of preparing europium-activated yttrium vanadate, YVOnEu, as described in U.S. Patent No. 3,l52,085 issued Oct. 6, l964.

The europium concentrations can vary between 0.l mole percent and I0 mole percent. The optimum concentrations have been found to be in the 0.5 to 7 percent range with some of the best results being obtained with l percent Eu. This is lower than most other europium-activated phosphors such as YVO :Eu, Y O zEu and Y O S Eu which require 4 percent or more to provide good brightness, but which change colors with lower concentrations. Since europium compounds are quite costly, it is advantageous and more efiicient to use smaller concentrations.

The following are some examples of starting compositions for preparing LiInO :Eu using different concentrations of Eu.

{Grams} LiIno.ts01:0.05Eu LiIno.950 :0.05Eu LiInQ OgKLOIEu 0. 74 LiaCOa 1. 38 LiNOa 1. 38 LiNO: 2. 64 ImOg 5. 72 ID(NO3)3 2. 78 11110: 0. 176 E1110: 0. 176 Ellzoa 0. 035 E1110:

The examples below are all for LiIn O- :0.01Eu

As shown in FIG. I, the novel phosphor is coated onto a faceplate 10 of a cathode-ray tube 12 in any suitable manner, such as by known settling, slurry or dusting techniques, to form a luminescent screen 14 or portion thereof. When excited by an electron beam 16, the phosphor luminesces in a bright red color having several low-level peaks l8 and one narrow high-level emission band 20, as indicated in the spectral response curve of Relative Intensity versus Wavelength in nanometers, as shown in FIG. 2. For test purposes, the material was arranged in a holder within a specially built cathode-ray tube and was subjected to an electron beam current of 0.5

microampii/cm. with ID kilovolts applied between the clec- Chromlticity diagram Approximate Phosphor coordinates lumen equivalent.

at y lumens/rad. watt cd. ,zn, ,s:A; 0.66 0.34 so YVO.:Eu 0.67 0.33 250 ;Y,O,:Eu 0.64 0.36 305 Y,0,S:Eu 0.66 0.34 255 LilnO,:Eu 0.65 0.35 330 The red-emitting phosphors are readily combined in a known manner with the other primary green and blue color phosphors in a desired pattern to form a screen for a color television tube.

As described in the aforementioned US. Patent No. 3,243,625, europium-activated compounds provide an improved efficiency, linearity and color stability over cadmium and zinc sulfide phosphors. In addition, as indicated in the table of red-emitting materials. the novel LilnO zEu composition of the present invention provides a higher luminous output in comparison with other similarly activated compounds. as well as good stable red color characteristics upon excitation by an electron beam.

While only a single embodiment has been illustrated, it is to be understood that the invention is not limited to the exact form or use shown and that other variations may be made in the particular configuration and material without departing from the scope of the invention as set forth in the appended claims. i

What is claimed is: 1

l. A cathode-ray tube comprising an evacuated envelope having a faceplate at one end, a phosphor screen on the inner surface of said faceplate, an electron gun at the other end of said envelope, said screen including a phosphor of europiumactivated lithium indate having the formula LilnO,:Eu which emits a red color in the visible region of the spectrum upon excitation by an electron beam from said gun.

2. The device of claim 1 wherein said phosphor is activated by between 0.1 and 10 mole percent europium and has relatively stable color characteristics in that range. 

2. The device of claim 1 wherein said phosphor is activated by between 0.1 and 10 mole percent europium and has relatively stable color characteristics in that range. 